So, MgF2 has a boiling point of 4100 F. That's pretty hot. So you'd need to achieve this temperature and get the gas inside a container with your lens, like cigarette smoke on your windows. Hmm... I need to think this thing through. A bell jar seems easy enough to come up with. A vacuum pump could be the intake side of any air compressor. I saw a fancy machine on ebay for several thousand dollars, but certainly don't have a pile of cash I can peel off a few bills from. Certainly this can be done at home.